1. Field of the Invention
The present invention relates to a method and apparatus for connecting networks suitable for enabling signal transmission even when the network system is configured between equipment connected with a line transmission system such as IEEE 1394, which is capable of an isochronous transmission of signals from a plurality of equipment, and equipment connected with a radio transmission system, different from the former system.
2. Related Art Statement
Recently, digital processing of images has been discussed. Generally, digitization of video signals generates a large amount of information, and transmitting or recording this information without compressing it is difficult because of transmission speed, cost or the like. Consequently, in transmitting or recording video signals, some technique to compress images is indispensable, and various plans for the standardization of techniques have been discussed. For animations, the MPEG (Moving Picture Experts Group) system has become standardized.
Especially, the MPEG2 system is most widely used as a standardized system for image compressing, and it has been adopted for digital broadcasting in the United States and Europe. Decoders corresponding to this MPEG standard as merchandise have been supplied as the MPEG decode board and mounted on computers and the like.
With the improvement in image compressing techniques, development in equipment for the digital image has also progressed. Not only have products like the digital VTR been introduced but other merchandise like the decoder for digital broadcasting (digital set top box), the digital video disc player (hereinafter referred to as DVD) and the like have been developed.
As digitization reduces degradation in transmission and recording of images, it advantageously regenerates images in high quality. Considering this advantage, a preferable configuration for each piece of digital video equipment would be that it have a digital interface which enables not only the conventional analogue inputting/outputting but also the inputting/outputting of digital signals as they are. With the digital interface provided, video data can be dealt with simply as digital data. Mutual conversion becomes possible not only for video equipment, but also for computers, enabling data transmission.
For apparatus provided with digital interface, in order to enable mutual data transfer with all the digital video equipment and computers, adoption of a unified interface system has been considered. To make it usable not only for digital video equipment but also for the computer system, the adoption of the standards of SCSI or RS232 is possible, for example. However, since the transmission rate of SCSI or RS232 is extremely low, it is impossible to transmit video data which require a transmission rate over several Mbps (bit/second). Besides, video data, unlike computer data, need to be transmitted periodically (also called isochronous transmission) in real time. These interface systems cannot be adopted for video transmission.
This being the case, at present, a high-speed interface system suitable for video data is under consideration by the council of digital interface VTR and at R4.1 of E1A, a U.S. council of TV (Advanced TV) decoder. Particularly, the IEEE (Institute of Electrical and Electronics Engineers) 1394 system, which has the isochronous transfer (also called synchronous transfer) function, is being adopted as the post SCSI system.
On this IEEE 1394 system, the 1394 Trade Association (also called 1394TA) has taken a leading part in the work to standardize and extend the system. This transmitting system, enables, at the same time, isochronous communication and is especially effective for video transmission. Consequently, AV apparatus manufacturers also participate in the standardizing work positively.
Such anticipated IEEE 1394 system is described in detail on pages 152 to 163 under the title of `Comparison of Three New Interfaces, searching design concept for post-SCSI` of Nikkei Electronics 1994. 7. 4 (No. 612) (Reference 1). As reported on and after page 161 of the article, the basic use of the IEEE 1394 is for computers, but because it provides the isochronous transfer function provided, this system is also more effective for video data than other interface systems. Data of animation or sound are transferred by this system periodically without fail, and regenerated data never become strained.
The IEEE 1394 also has a function to set topology automatically (refer to pages 155 to 159 under the title of `Automatic Setting of Topology` of Reference 1). IEEE 1394 resets the following; the connection of devices, confirmation of connection status of each device at non-connected time or at the time of power-source-throwing-in, setting of membership among devices and ID setting of each device. That is, in contrast to the topology of SCSI which can be only daisy-chained, the IEEE 1394's can also form a tree configuration to connect a plurality of devices.
Suppose the IEEE 1394, which is particularly effective for video transmission as described above, is standardized and put to practical use, it is possible to configure a network connection of a plurality of devices connected by a transmission configuration such as IEEE 1394, and different transmission configuration to form a network system. In other words, it would be a network system configured by connecting two different protocols.
In such a network system, if one side is a plurality of devices connected in the IEEE 1394 transmission configuration, to the other side, a plurality of devices are connected in another transmission configuration of a different protocol. In this case, in order to connect two different protocols, one device out of the plurality of devices must be connected by two different protocols. That is, the device to be connected in such a way is required to perform signal transmission in both protocols.
However, ordinary connecting devices are not provided with such a function for performing signal transmission in both protocols. In brief, even though network connection is done, real time signal transmission to both sides is impossible.
For example, in the IEEE 1394, performing arbitration, by using the automatic setting function of topology, instantly determines the membership among various devices; out of a plurality of connected devices, one becomes parent and other devices become children. At this time, if a device which is connected by a different protocol is the parent device on the IEEE 1394 side, the parent device and devices connected by the other protocol are connected by a transmission configuration other than the IEEE 1394.
Assume that the radio system protocol that is adopted as a transmission system is something other than IEEE 1394. While, in the IEEE 1394, the high-speed real time transmission of animation data is possible at the maximum transmission capability of 100-400 Mbps, the transmission capability of the radio transmission system is extremely low, a mere several Mbps. Consequently, while real time data transmission is possible with the devices connected in the IEEE 1394, real time data transmission from the parent device to other devices through radio system protocol is impossible because of the differences in transmission capability and in the packet size for transmission.
Further, as described above, in addition to the difference in protocols on both sides when the devices are connected in different transmission configuration on each side, the controlling method to control the node ID (identification number allocated to each device) also differs.
For these reasons, ordinary methods of and apparatus for connecting networks are incapable of two-way real time transmission of data as they are. When a network is configured by connecting the IEEE 1394 (line transmission system) with the radio transmission system of a different protocol, there are differences in protocol on both sides, as well as differences in the methods to control the node ID, and differences in the packet size at the time of data transmission. Thus, there has been a problem for ordinary methods and apparatus for connecting networks, because that a network system using the IEEE 1394 and the radio transmission system cannot be configured.